Actuators, as control valves, affecting, in particular controlling by open-loop control and/or close-loop control, the process medium stream of the process engineering plant can take on critical operating conditions. In case of control valves, these critical operating conditions are for example leakage or cavitation which are accompanied by a characteristic sound emission. Sound emissions of the actuator can be acoustically recorded and evaluated. An early detection of actuator damages has to be achieved to avoid a complete failure of the actuator or an interruption of the plant operation as for example due to acute maintenance or repair works on the process engineering plant.
A diagnostic system for control valves and close valves of a process engineering plant is described in EP 0 637 713 A1. The diagnostic system uses an electric processor of a positioner that is connected with a structure-borne sound sensor that monitors an upper valve part and generates measured values for determining cavitation. In case of an impermissible deviation of the measured sound values from reference measured values being recorded before during proper operation of the positioner, an alarm signal is emitted. Due to the direct arrangement of the structure-borne sound sensor at the upper valve part, the diagnostic system is susceptible for false alarms since the source of the sound can only be identified with an increased diagnostic effort.
A diagnostic system for a positioner of a control valve of a process engineering plant is described in DE 199 24 377 B4. In this document, a structure-borne sound sensor is mounted directly at the outside of the control valve housing. A structure borne sound spectrum is recorded when the control valve is slightly open and compared during operation when the control valve is closed. After comparison of both measured values a conclusion regarding the tightness or the leakage of the control valve can be made. Also with this diagnostic system a high reliability is not to be assumed.
In EP 1 216 375 B2, it is proposed to mount a sound sensor to the yoke, i.e. to a connection flange between the actuating drive housing and the control valve housing, to a flange between the control valve housing and the plant conduit, or to the valve rod itself. Mounting the sound sensor to the valve rod bears the advantage that especially high-frequency sound signal parts of adjacent components in the process engineering plant which are for example transmitted through the conduit are muffled and attenuated. The structure-borne sound of the control valve to be monitored that characterizes the operation can be scanned directly via the closing body and thus via the valve rod. It is further proposed to use a piezoelectric transducer element for the sound sensor that is designed insensitive in the area of low frequency operating noise and sensitive in the area of higher-frequency operating noise. This is based on the knowledge that different operating conditions, in particular operating error types, show characteristic derivations from specific frequency bands. The known diagnostic system suffers from a complex signal evaluation and a low reproducibility.
A diagnostic device for a field device in the industrial environment of a procedure engineering plant is described in WO 02/077733 A1. The field device is connected via a data path with a central controller and comprises an acoustic data recording device in form of a microphone. In case of derivation of the noise information to stored data of a central controller the functionality of the field device can be verified. This diagnostic device has the drawback of low measurement accuracy.
DE 10 2006 055 747 A1 describes a diagnostic system for an actuator that is operated via a pneumatic actuating drive which is controlled by a positioner. For checking the correct functionality of the control valve, it is known that specific acoustic signals are emitted during the intended use. These should be recorded in the pneumatic actuating drive, from which conclusions about the state of the control valve should be made. This is based on the knowledge that acoustic signals are transmitted to the diaphragm of the pneumatic actuating drive, which acoustic signals are amplified by the large diaphragm and passed to the air pressure medium. As acoustic sensor a pressure sensor is used. The diagnostic system also suffers from a low reliability and a high diagnostic effort to filter out the essential signals from the nonessential signals in view of the assessment of the functionality of the control valve.
A method for the diagnosis of a pneumatically operated control valve is described in DE 10 2006 059 938 B3. A positioner controls the pneumatic auxiliary energy which is fed to an actuating drive. The positioner has a nozzle-flapper arrangement which is additionally used as sensor. Such a diagnostic system is temperature-dependent and thus rather unsuitable for the use in the process engineering plant.
EP 2 191 182 B1 describes a structure-borne sound sensor which is integrated in a fastening screw which mounts the positioner to the field device. For identification of a leakage at the actuating drive, the pressure of the process fluid is measured at the inlet side and compared with a reference pressure. The diagnostic system triggers an alarm when the measured structure-borne sound level exceeds a reference value and at the same time a pressure measured value is smaller than the reference pressure value. Precise statements about a leakage or cavitation cannot be obtained with the known system.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.